1. Field of the Invention
The present invention generally relates to a device and a method of screening for individual balls such as solder balls.
2. Description of Related Art
In manufacturing metal balls such as solder balls, in addition to individual balls with a specified sphericity (referred to as individual balls hereinafter), balls formed with a plurality of balls joined together (referred to as conjoined balls hereinafter) and balls with an abnormal shape such as elliptical balls (referred to as abnormal balls hereinafter) are formed. One of the quality items required by the metal balls is the shape. Typically, the conjoined balls and the abnormal balls are unqualified products, so they should be filtered and removed.
For the technique for screening for the conjoined balls and the abnormal balls from individual balls, conjoined balls, and abnormal balls, a device (referred to as slope rolling hereinafter) is disclosed in patent document 1, in which a collection of individual balls, conjoined balls, and abnormal balls roll down a slope that is elevated at an angle at the top, the balls are screened for according to the speed difference or the direction difference thereof, and the individual balls are recovered into a recovering container disposed at the lower end of the slope. The screening is achieved through the advantage that the individual balls descend along the slope, while the conjoined balls or the abnormal balls descend with large amplitude, wobbling like falling leaves. Accordingly, with such descending behaviours, by adjusting the slope width and the distance from the lower end of the slope to the individual balls recovering container, the conjoined balls and the abnormal balls may descend to the right or left of the slope or the front of the recovering container, and the individual balls to be recovered are recovered in the recovering container.
Also, in the patent document 1, a method for tilting the descending direction of the balls or tilting the descending direction of the balls along a right-angle direction is also disclosed.
On the other hand, a scheme is provided in patent document 2, in which a groove structure is disposed between a supply device for supplying the mixture of individual balls, conjoined balls, and abnormal balls and a slope for descending the mixture down from the supply device, and the conjoined balls and the abnormal balls are stopped at the groove structure due to a chock effect resulted from the groove structure, thereby improving the screening precision. The advantage of the proposal lies in that the individual balls with a diameter of about 300 μm can be precisely screened for.
[Patent document 1] Japanese Laid-Open Patent Publication No. 11-319728.
[Patent document 2] Japanese Laid-Open Patent Publication No. 11-319722.
The inventor discovers through a research that, when the individual balls are screened for and recovered from a mixture of individual balls, conjoined balls, and abnormal balls by utilizing a slope descending mechanism of the patent document 1 or 2, the mixture of individual balls, conjoined balls, and abnormal balls may sometimes be stuck on the slope or groove structure. Therefore, the rolling action thereof on the slope or in groove structure is blocked. The smaller the size of the individual balls, conjoined balls, and abnormal balls are being screened for, the more obvious the problem appears. When the ball size is less than or equal to 200 μm, and the diameter is below 100 μm, the balls could hardly roll down, so the screening cannot be accomplished.